Alcohols have known potential for use as an alternative fuel for transportation vehicles. In addition to the realization that the world's petroleum reserves are finite and have only a relatively short life at present rates of consumption, environmental concerns and national security issues also serve to promote the use of alcohol (mainly ethanol and methanol) as a clean burning, high octane gasoline extender or replacement. Methanol is perhaps the most likely candidate to replace crude oil based liquid fuels.
The establishment of alcohol fuel distribution systems which provide a wide geographical spread of outlets will take considerable time to develop, as will replacement of automobiles and other transportation vehicles powered by engines designed to burn only gasoline. This conversion or transition to alcohol fuel, such as neat methanol fuel, would be facilitated by a methanol fueled vehicle that can operate satisfactorily using gasoline, methanol (or other alcohol) or any mixture of gasoline and alcohol(s). However, spark ignited engines require for proper combustion an air-fuel ratio for gasoline which is substantially different than the air-fuel ratio for methanol or ethanol. The problem is further complicated by mixtures of gasoline and alcohol which have a stoichiometric ratio that varies with the composition of the mixture.
Accordingly, a need exists for a sensor that may be used in a fuel system of an automobile or other vehicle that can measure the proportion of alcohol in the fuel and provide a representative signal for controlling the air-fuel ratio. The function of the fuel system would be to control the quantity of fuel delivered based on the mass of air inducted by the engine and the fuel composition as detected by the sensor. Auto makers have developed various mechanisms and controls for adjusting the air-fuel ratio in response to a signal from a fuel composition sensor. Most likely such adjustment would be controlled by an on board computer programmed to adjust automatically the air-fuel ratio in response to a signal from the fuel composition sensor. The computer might also control other operating parameters of the engine in response to the sensor output such as ignition timing.
Several infrared fuel composition sensors have been developed and/or proposed. U.S. Pat. No. 4,594,968 describes a device for determining the composition of an alcohol-gasoline mixture for use in the automatic regulation of engines fed with fuel mixtures having a variable alcohol content. The sensor uses a diode emitting a light beam in the infrared range having an average wavelength of 940 nanometers. The beam is divided into two parts by a semi-reflecting separating plate. One part of the beam passes to a first phototransistor through a measuring cell while the other part of the beam passes to a second phototransistor through a reference cell containing a reference liquid of known composition such as pure methanol. The phototransistors are connected to the two inputs of a differential amplifier which provides a measurement signal assumed to be representative of the methanol percentage.
In addition to the above noted sensor based on infrared absorbance, other proposed or investigated sensors have been based on index of refraction, ultrasonic sound or index of refraction. These prior art sensors suffer from one or more drawbacks including sensitivity to temperature, fuel flow, impurities, mixture inhomogeneities, environmental temperature constrains, reliability, complexity and cost.